Plastic lenses for spectacles

ABSTRACT

Plastic lenses for spectacles having a high refractive index and a high Abbe&#39;s number, and having good transparency, heat resistance, mechanical strength and impact resistance are produced. Plastic lenses for spectacles are produced through polymerization of an epithio group-having compound (a), a polythiol compound (b) and a polyisocyanate compound (c) whereby the ratio (by mol) of the polythiol compound (b) to the polyisocyanate compound (c) is at least 1.2 calculated on the basis of —SH and —NCO groups. The invention also provides a process for preparing such lenses and a catalyst for use in said process.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to plastic lenses for spectacles,in particular to those having a high refractive index and a high Abbe'snumber and having good transparency, heat resistance, mechanicalstrength and impact resistance.

[0002] Plastics are being much used these days for optical products suchas lenses, as being lightweight, hardly broken and easily colored whencompared with glass. It is known to use, as the monomer material, anepithio group-having compound in producing plastic lenses. For example,Japanese Patent Laid-Open No. 180977/1999 discloses a plastic lenshaving optical properties, a refractive index of 1.70 or so and anAbbe's number of from 35 to 37 or so, which, however, still leaves roomfor improving its heat resistance and mechanical strength.

[0003] Japanese Patent Laid-Open Nos. 292950/1999 and 352302/1999disclose a resin composition prepared by reacting an epithiogroup-having compound, an isocyanate group-having compound and a thiolgroup-having compound in a ratio of thiol group/isocyanate group of atmost 1, and its use for plastic lenses. They say that the resincomposition has improved heat resistance, mechanical strength and impactresistance. However, the plastic lenses of the resin composition areoften cloudy and could not be well cured, and are therefore unsuitableto practical use for spectacles.

[0004] The present invention has been made so as to solve the problemsas above, and to provide plastic lenses for spectacles having a highrefractive index and a high Abbe's number and having good transparency,heat resistance, mechanical strength and impact resistance.

SUMMARY OF THE INVENTION

[0005] The present inventors have found that (1) a plastic lens producedthrough polymerization of an epithio group-having compound (a), amercapto (—SH) group-having polythiol compound (b) and an isocyanate(—NCO) group-having, sulfur-free polyisocyanate compound (c_(o)) in aratio (by mol), —SH/—NCO, of at least 2.0; (2) a plastic lens producedthrough polymerization of an epithio group-having compound (a), amercapto group-having polythiol compound (b) and an isocyanategroup-having, sulfur-containing polyisocyanate compound (c_(s)) in aratio (by mol), —SH/—NCO, of at least 1.2 can attain the desired balanceof properties.

DETAILED DESCRIPTION OF THE INVENTION

[0006] Specifically, the first aspect of the invention is to provide aplastic lens for spectacles, which is produced through polymerization ofan epithio group-having compound, a polythiol compound and a sulfur-freepolyisocyanate compound in a ratio (by mol) of the polythiol compound tothe sulfur-free polyisocyanate compound, —SH/—NCO, of at least 2.0.

[0007] The second aspect of the invention is to provide a plastic lensfor spectacles, which is produced through polymerization of an epithiogroup-having compound, a polythiol compound and a sulfur-containingpolyisocyanate compound in a ratio (by mol) of the polythiol compound tothe sulfur-containing polyisocyanate compound, —SH/—NCO, of at least1.2.

[0008] The polyisocyanate compound (c) may be a mixture of asulfur-containing polyisocyanate compound (c_(s)) and a sulfur-freepolyisocyanate compound (c_(o)). The plastic lens material may beproduced through polymerization of at least one polyisocyanate compoundselected from di(isocyanatomethyl)bicycloheptane,bis(isocyanatomethyl)-1,4-dithian and dicyclohexylmethane diisocyanate,and an epithio group-having compound and a polythiol compound. Theplastic lens material may also be produced through polymerization of apolyisocyanate compound prepared by mixingbis(isocyanatomethyl)-1,4-dithian withdi(isocyanatomethyl)bicycloheptane and/or dicyclohexylmethanediisocyanate, and an epithio group-having compound and a polythiolcompound.

[0009] In its first aspect, the plastic lens for spectacles of theinvention is produced through polymerization of an epithio group-havingcompound, a polythiol compound and a sulfur-free polyisocyanate compoundin a ratio (by mol) of the polythiol compound to the sulfur-freepolyisocyanate compound, —SH/—NCO, of at least 2.0. If the ratio—SH/—NCO is smaller than 2.0, the effect of the first aspect of theinvention cannot be obtained.

[0010] In its second aspect, the plastic lens for spectacles of theinvention is produced through polymerization of an epithio group-havingcompound, a polythiol compound and a sulfur-containing polyisocyanatecompound in a ratio (by mol) of the polythiol compound to thesulfur-containing polyisocyanate compound, —SH/—NCO, of at least 1.2. Ifthe ratio —SH/—NCO is smaller than 1.2, the effect of the inventioncannot be obtained. Polyisothiocyanate group-having compounds areoutside the scope of the sulfur-containing polyisocyanate compound, asnot producing the advantages of the invention.

[0011] With the increase in the proportion of thiourethane bondstherein, lenses for spectacles could have an increased tensile strengthcharacteristic of polythiourethanes, but their refractive index couldnot increase. For further increasing the refractive index of the lenseswhile producing lenses having an increased tensile strength and anincreased Abbe's number, heretofore proposed is adding an epithiogroup-having compound to the lenses. However, in the system for lensesthat comprises an epithio group-having compound, a polythiol compoundand a sulfur-free polyisocyanate compound, if the polyisocyanate isexcessive over the polythiol, as in the Examples in Japanese PatentLaid-Open Nos. 180977/1999 and 292950/1999, the lenses formed arecloudy.

[0012] On the other hand, lenses formed from a sulfur-containingpolyisocyanate compound, an epithio group-having compound and apolythiol compounds are not cloudy even when the proportion of thesulfur-containing polyisocyanate compound therein is increased. Sincethe sulfur content and the thiourethane bond content thereof are high,the lenses have an increased refractive index and their mechanicalstrength and Abbe's number are not lowered.

[0013] Some sulfur-containing polyisocyanate compounds such asbis(isocyanatomethyl)-1,4-dithian are solid at room temperature. Such asolid, sulfur-containing polyisocyanate compound may be dissolved in asulfur-free polyisocyanate compound that is liquid at room temperature.As being liquid at room temperature, the resulting mixture is easy tohandle when it is used in producing lenses. In mixing the two, the ratioof the sulfur-containing polyisocyanate compound to the sulfur-freepolyisocyanate compound may be so controlled that the liquid,sulfur-free polyisocyanate compound can dissolve the solid,sulfur-containing polyisocyanate compound to give a mixture that isliquid at room temperature. In practice, the ratio of thesulfur-containing polyisocyanate compound and the sulfur-freepolyisocyanate compound may be such that the amount of the sulfur-freepolyisocyanate compound is enough for dissolving therein thesulfur-containing polyisocyanate compound which is solid at roomtemperature. The ratio of the two can be readily determined to producelenses having the desired balance of properties. For example, from 5 to30 parts by weight of a sulfur-free polyisocyanate may be added to 10parts by weight of a sulfur-containing polyisocyanate.

[0014] A sulfur-free polyisocyanate compound that can be used in thefirst and second aspects of the invention isdi(isocyanatomethyl)bicycloheptane and/or dicyclohexylmethanediisocyanate. A sulfur-containing polyisocyanate compound that can beused is bis(isocyanatomethyl)-1,4-dithian.

[0015] The lens for spectacles may be produced through polymerization ofat least one polyisocyanate compound selected fromdi(isocyanatomethyl)bicycloheptane, bis(isocyanatomethyl)-1,4-dithianand dicyclohexylmethane diisocyanate, and an epithio group-havingcompound and a polythiol compound. The lens for spectacles may also beproduced through polymerization of a polyisocyanate compound prepared bymixing bis(isocyanatomethyl)-1,4-dithian withdi(isocyanatomethyl)bicycloheptane and/or dicyclohexylmethanediisocyanate, and an epithio group-having compound and a polythiolcompound.

[0016] Bis(isocyanatomethyl)-1,4-dithian is solid at room temperature.When it is mixed with di(isocyanatomethyl)bicycloheptane and/ordicyclohexylmethane diisocyanate, and dissolved therein, the resultingmixture is liquid at room temperature, and is easy to handle.

[0017] The ratio (by mol) of the polythiol compound to thepolyisocyanate compound, —SH/—NCO, is preferably at least 2.0, morepreferably at least 2.5.

[0018] The polythiol compound for use in the invention includes, forexample, aliphatic thiols such as methanedithiol, 1,2-ethanedithiol, 1,1-propanedithiol, 1,2-propanedithiol, 1,3-propanedithiol,2,2-propanedithiol, 1,6-hexanedithiol, 1,2,3-propanetrithiol,tetrakis(mercaptomethyl)methane, 1,1-cyclohexanedithiol,1,2-cyclohexanedithiol, 2,2-dimethylpropane-1,3-dithiol,3,4-dimethoxybutane-1,2-dithiol, 2-methylcyclohexane-2,3-dithiol, 1,1-bis(mercaptomethyl)cyclohexane, bis(2-mercaptoethyl)thiomalate,(2-mercaptoethyl)2,3-dimercaptosuccinate, 2,3-dimercapto-1-propanol(2-mercaptoacetate), 2,3-dimercapto-1-propanol (3-mercaptoacetate),diethylene glycol bis(2-mercaptoacetate), diethylene glycolbis(3-mercaptopropionate), 1,2-dimercaptopropyl methyl ether,2,3-dimercaptopropyl methyl ether,2,2-bis(mercaptomethyl)-1,3-propanedithiol, bis(2-mercaptoethyl)ether,ethylene glycol bis(2-mercaptoacetate), ethylene glycolbis(3-mercaptopropionate), trimethylolpropane tris(2-mercaptoacetate),trimethylolpropane tris(3-mercaptopropionate), pentaerythritoltetrakis(2-mercaptoacetate), pentaerythritoltetrakis(3-mercaptopropionate),1,2-bis(2-mercaptoethylthio)-3-mercaptopropane; aliphatic thiols havingsulfur atom(s) in addition to mercapto groups, such asbis(mercaptomethyl)sulfide, bis(mercaptoethyl)sulfide,bis(mercaptopropyl)sulfide, bis(mercaptomethylthio)methane,bis(2-mercaptoethylthio)methane, bis(3-mercaptopropyl)methane,1,2-bis(mercaptomethylthio)ethane, 1,2-(2-mercaptoethylthio)ethane,1,2-(3-mercaptopropyl)ethane, 1,3-bis(mercaptomethylthio)propane,1,3-bis(2-mercaptoethylthio)propane,1,3-bis(3-mercaptopropylthio)propane,1,2-bis(2-mercaptoethylthio)-3-mercaptopropane,2-mercaptoethylthio-1,3-propanedithiol,1,2,3-tris(mercaptomethylthio)propane,1,2,3-tris(2-mercaptoethylthio)propane,1,2,3-tris(3-mercaptopropylthio)propane,tetrakis(mercaptomethylthiomethyl)methane,tetrakis(2-mercaptoethylthiomethyl)methane,tetrakis(3-mercaptopropylthiomethyl)methane,bis(2,3-dimercaptopropyl)sulfide, 2,5-dimercapto-1,4-dithian,bis(mercaptomethyl)disulfide, bis(mercaptoethyl)disulfide,bis(mercaptopropyl)disulfide, and their thioglycolates andmercaptopropionates, hydroxymethylsulfide bis(2-mercaptoacetate),hydroxymethylsulfide bis(3-mercaptopropionate), hydroxyethylsulfidebis(2-mercaptoacetate), hydroxyethylsulfide bis(3-mercaptopropionate),hydroxypropylsulfide bis(2-mercaptoacetate), hydroxypropylsulfidebis(3-mercaptopropionate), hydroxymethyidisulfidebis(2-mercaptoacetate), hydroxymethyldisulfidebis(3-mercaptopropionate), hydroxyethyldisulfide bis(2-mercaptoacetate),hydroxyethyldisulfide bis(3-mercaptopropionate), hydroxypropyidisulfidebis(2-mercaptoacetate), hydroxypropyldisulfidebis(3-mercaptopropionate), 2-mercaptoethyl-ether bis(2-mercaptoacetate),2-mercaptoethyl-ether bis(3-mercaptopropionate), 1,4-dithian-2,5-diolbis(2-mercaptoacetate), 1,4-dithian-2,5-diol bis(3-mercaptopropionate),bis(2-mercaptoethyl)thioglycolate, bis(2-mercaptoethyl)thiodipropionate,bis(2-mercaptoethyl)4,4′-thiodibutyrate,bis(2-mercaptoethyl)dithiodiglycolate,bis(2-mercaptoethyl)dithiodipropionate,bis(2-mercaptoethyl)4,4′-dithiodibutyrate,bis(2,3-dimercaptopropyl)thiodiglycolate,bis(2,3-dimercaptopropyl)thiodipropionate,bis(2,3-dimercaptopropyl)dithiodiglycolate,(2,3-dimercaptopropyl)dithiodipropionate,4-mercaptomethyl-3,6-dithiaoctane-1,8-dithiol,bis(mercaptomethyl)-3,6,9-trithia-1,1,1-undecanedithiol,bis(1,3-dimercapto-2-propyl)sulfide; and heterocyclic compounds havingsulfur atom(s) in addition to mercapto groups, such as3,4-thiophenedithiol, tetrahydrothiophene-2,5-dimercaptomethyl,2,5-dimercapto-1,3,4-thiadiazole, 2,5-dimercapto-1,4-dithian,2,5-dimercaptomethyl-1,4-dithian.

[0019] The polythiol compound for use in the invention is preferablybismercaptomethyl-1,4-dithian and/or(4-mercaptomethyl-2,5-dithianyl)methyl disulfide.

[0020] The epithio group-having compound for use in the invention is anepisulfide monomer, including, for example, alicyclic skeleton-havingepisulfide compounds such as 1,3- and1,4-bis(β-epithiopropylthio)cyclohexanes, 1,3- and1,4-bis(β-epithiopropylthiomethyl)cyclohexanes,bis[4-(β-epithiopropylthio)cyclohexyl]methane,2,2-bis[4-(β-epithiopropylthio)cyclohexyl]propane,bis[4-(β-epithiopropylthio)cyclohexyl]sulfide; aromatic skeleton-havingepisulfide compounds such as 1,3- and1,4-bis(β-epithiopropylthio)benzenes, 1,3- and1,4-bis(β-epithiopropylthiomethyl)benzenes,bis[4-(β-epithiopropylthio)phenyl]methane,2,2-bis[4-(β-epithiopropylthio)phenyl]propane,bis[4-(β-epithiopropylthio)phenyl]sulfide,bis[4-(β-epithiopropylthio)phenyl]sulfine,4,4-bis(β-epithiopropylthio)biphenyl; dithian skeleton-having episulfidecompounds such as 2,5-bis(β-epithiopropylthiomethyl)-1,4-dithian,2,5-bis(β-epithiopropylthioethylthiomethyl)-1,4-dithian,2,5-bis(β-epithiopropylthioethyl)-1,4-dithian,2,3,5-tri(β-epithiopropylthioethyl)-1,4-dithian; and aliphaticskeleton-having episulfide compounds such as2-(2-β-epithiopropylthioethylthio)-1,3-bis(β-epithiopropylthio)propane,1,2-bis[(2-β-epithiopropylthioethyl)thio]-3-(epithiopropylthio)propane,tetrakis(β-epithiopropylthiomethyl)methane,1,1,1-tris(β-epithiopropylthiomethyl)propane,bis(β-epithiopropyl)sulfide.

[0021] Many epithio group-having compounds are known, and their examplesare described in, for example, Japanese Patent Laid-Open Nos.071580/1997, 110979/1997, 255781/1997, 081320/1991, 140070/1999,183702/1999, 189592/1999, 180977/1999, and Japanese Patent Re-Laid-OpenNo. 810575/1989. Episulfide monomers disclosed in these publications areall usable in the invention.

[0022] Of the epithio group-having compounds mentioned hereinabove,bis(β-epithiopropyl)sulfide is preferred for use in the invention.

[0023] Plastic lenses for spectacles of the invention preferably containfrom 60 to 85% by weight of the epithio group-having compound and from15 to 40% by weight of the polythiol compound and the balancepolyisocyanate compound. Plastic lenses are produced that have arefractive index of 1.70 or so and an Abbe's number of 36 or so andtheir mechanical strength is enough for practical use of the lenses inrimless spectacles.

[0024] Especially preferred combinations of the polyisocyanate compound,the polythiol compound and the epithio group-having compound for use inthe invention are mentioned below.

[0025] A. Di(isocyanatomethyl)bicycloheptane (polyisocyanate compound),bis(mercaptomethyl)-1,4-dithian (polythiol compound), andbis(β-epithiopropyl)sulfide (epithio group-having compound).

[0026] B. Di(isocyanatomethyl)bicycloheptane (polyisocyanate compound),(4-mercaptomethyl-2,5-dithianyl)methyl disulfide,bis(mercaptomethyl)-1,4-dithian (polythiol compounds), andbis(β-epithiopropyl)sulfide (epithio group-having compound).

[0027] C. Bis(isocyanatomethyl)-1,4-dithian (polyisocyanate compound),bis(mercaptomethyl)-1,4-dithian (polythiol compound), andbis(β-epithiopropyl)sulfide (epithio group-having compound).

[0028] D. Dicyclohexylmethane diisocyanate (polyisocyanate compound),(4-mercaptomethyl-2,5-dithianyl)methyl disulfide,bis(mercaptomethyl)-1,4-dithian (polythiol compounds), andbis(β-epithiopropyl)sulfide (epithio group-having compound).

[0029] E. Bis(isocyanatomethyl)-1,4-dithian anddi(isocyanatomethyl)bicycloheptane (polyisocyanate compounds),bis(mercaptomethyl)-1,4-dithian (polythiol compound), andbis(β-epithiopropyl)sulfide (epithio group-having compound).

[0030] F. Bis(isocyanatomethyl)-1,4-dithian and dicyclohexylmethanediisocyanate (polyisocyanate compounds), bis(mercaptomethyl)-1,4-dithian(polythiol compound), and bis(β-epithiopropyl)sulfide (epithiogroup-having compound).

[0031] The polymerization reaction according to the present inventionmay be performed in the presence of a catalyst. Any suitable catalystmay be employed. Specific examples of a suitable catalyst include aminessuch as aliphatic and aromatic tertiary amines, Lewis acids and dialkyltin salts such as dibutyltin dichloride and dibutyltin dilaurate.

[0032] Preferably, a catalyst of the following general formula (1) isadded to the plastic lenses for spectacles of the invention while theyare produced through polymerization. The catalyst facilitates theproduction of lenses with neither optical strain nor striae from thepolymerizing compounds, epithio group-having compound, polythiolcompound and polyisocyanate compound.

[0033] wherein R indicates an alkyl group having from 1 to 4 carbonatoms. The catalyst of formula (1) is preferably at least one selectedfrom tetramethyldiacetoxy-distannoxane,tetraethyldiacetoxy-distannoxane, tetrapropyldiacetoxy-distannoxane andtetrabutyldiacetoxy-distannoxane.

[0034] In case where the plastic lenses of the invention are, afterproduced through polymerization in a mold, difficult to release from themold, any known external and/or internal lubricant may be used for themor may be added to them to thereby improve the releasability of thelenses. If desired, an UV absorbent may be added to the lenses whilethey are produced. This is for protecting the resin of the lenses fromUV rays and also for protecting the eye from them, and its amount to beadded to each lens may fall generally between 0.03% and 3% or so, thoughdepending on its UV absorbability and on the maximum wavelength of theUV rays to be absorbed by it. As the case may be, the lenses producedmay be dipped in an UV absorbent. The plastic lenses of the inventionare easy to color with a colorant.

[0035] For improving their scratch resistance, the plastic lenses of theinvention may be coated with a hard film, for which is usable a coatingliquid that contains an organosilicon compound or acrylic compound andinorganic fine particles of, for example, tin oxide, silicon oxide,zirconium oxide or titanium oxide. For improving their impactresistance, the plastic lenses may be coated with a primer layer thatconsists essentially of polyurethane.

[0036] For preventing them from glaring, the hard film of the plasticlenses may be further coated with an anti-glare film that contains aninorganic substance of, for example, silicon oxide, titanium dioxide,zirconium oxide or tantalum oxide. For improving their water repellency,the anti-glare film of the plastic lenses may be coated with awater-repellent film of an organic fluorosilicon compound.

[0037] The plastic lenses for spectacles of the invention mentionedhereinabove have a refractive index of from 1.65 to 1.76, and theirmechanical strength is higher than that of conventional plastic lenses.In addition, they are highly transparent, and have good heat resistanceand impact resistance.

[0038] The invention is described in more detail with reference to thefollowing Examples, which, however, are not intended to restrict thescope of the invention. The physical properties of the plastic lensesfor spectacles produced in the following Examples and ComparativeExamples are measured according to the methods mentioned below.

[0039] (1) Refractive Index and Abbe's Number

[0040] Measured at 20° C. with a precision refractometer, Kalnew'sKPR-200 Model.

[0041] (2) Heat Resistance

[0042] Measured with a thermal analyzer, Rigaku-sha's TAS-100, TMA8140,according to a penetration method (in which the thickness of the sampleanalyzed is 3 mm, the pin diameter is 0.5 mm, the load is 10 g, and theheating rate is 10° C./min). The peak temperature at which the thermalexpansion of the sample has changed is read, and this indicates the heatresistance of the sample.

[0043] (3) Tensile Strength

[0044] A lens prepared to have a power of 0.00 D, a diameter of 80 mmand a thickness of 1.8 mm is worked to be fittable to a frame of anelectronic universal tester (Yonekura Seisakusho's Model CATY200WR). Inthis, two holes each having a diameter of 1.6 mm are drilled at oppositeends. The thus-worked lens sample is fixed to the tester with 1.6 mmφpins fitted to its holes, and pulled at a stress rate of 0.05 mm/min,and the tensile strength at break of the sample is measured.

[0045] (4) Presence or Absence of Striae

[0046] Each sample is macroscopically checked as to whether or not ithas striae, according to the schlieren method. The samples thus checkedare evaluated on the basis of the following criteria:

[0047] 0: No striae found.

[0048] x: Striae found.

EXAMPLE 1

[0049]80.0 parts by weight of an epithio group-having compound,bis(β-epithiopropyl)sulfide, 15.65 parts by weight of a polythiolcompound, bis(mercaptomethyl)-1,4-dithian, and 4.35 parts by weight of apolyisocyanate compound, di(isocyanatomethyl)bicycloheptane were mixedwith stirring along with 50 ppm of an internal lubricant, a mixture ofdibutoxyethyl acid phosphate and butoxyethyl acid phosphate, to whichwere added 0.01 parts by weight oftetra-n-butyl-1,3-diacetoxy-distannoxane and 0.05 parts by weight oftetrabutylphosphonium bromide both serving as a catalyst. These werestirred and mixed under a reduced pressure of 10 mmHg for about 3minutes to prepare a monomer composition for lenses. The ratio of—SH/—NCO in this composition is given in Table 1.

[0050] Next, the monomer composition was cast into a glass mold forlenses (lens power 0.00 D, lens diameter 80 mm, lens thickness 1.8 mm),which had been previously prepared and equipped with a resin gasket. Themold was put into an electric furnace, and gradually heated therein atfrom 20° C. up to 100° C. over a period of 20 hours and then kept heatedat 100° C. for 30 minutes, through which the monomers were polymerized.

[0051] After having been thus polymerized, the lens was released fromthe mold, and then further heated at 110° C. for 1 hour. It was visuallychecked for outward appearance, and tested for (1) to (4) as above. Theresults are given in Table 1.

EXAMPLES 2 TO 10

[0052] Lenses were produced in the same manner as in Example 1, exceptthat the epithio group-having compound, the polythiol compound and thepolyisocyanate compound and their amounts were varied as in Table 1. Thelenses were visually checked for appearance, and tested for (1) to (4)as above. The results are given in Table 1.

COMPARATIVE EXAMPLE 1

[0053] 75.0 parts by weight of an epithio group-having compound,bis(β-epithiopropyl)sulfide, 5.0 parts by weight of a polythiolcompound, n-butyl thioglycolate, and 20.0 parts by weight of apolyisocyanate compound, m-xylene diisocyanate were mixed with stirring,to which was added 0.05 parts by weight of a catalyst,tetrabutylphosphonium bromide. These were stirred and mixed under areduced pressure of 10 mmHg for about 3 minutes to prepare a monomercomposition for lenses. The ratio of —SH/—NCO in this composition isgiven in Table 1.

[0054] Next, the monomer mixture was polymerized in the same manner asin Example 1. However, the product obtained was dark brown liquid andwas not resinous. Therefore, this was not tested for (1) to (4).

COMPARATIVE EXAMPLE 2

[0055] The monomers were polymerized in the same manner as inComparative Example 1, except that dimercaptoethyl sulfide was used inplace of n-butyl thioglycolate.

[0056] However, the product obtained was milky, opaque jelly. Therefore,this was not tested for (1) to (4).

COMPARATIVE EXAMPLE 3

[0057] 80.0 parts by weight of an epithio group-having compound,bis(β-epithiopropyl)sulfide, 5.0 parts by weight of a polythiolcompound, n-butyl thioglycolate, and 15.0 parts by weight of apolyisocyanate compound, isophorone diisocyanate were mixed withstirring, to which was added 0.05 parts by weight of a catalyst,tetrabutylphosphonium bromide. These were stirred and mixed under areduced pressure of 10 mmHg for about 3 minutes to prepare a monomercomposition for lenses. The ratio of —SH/—NCO in this composition isgiven in Table 1.

[0058] Next, the monomer mixture was polymerized in the same manner asin Example 1. However, the product obtained was dark brown liquid andwas not resinous. Therefore, this was not tested for (1) to (4).

COMPARATIVE EXAMPLE 4

[0059]80.0 parts by weight of an epithio group-having compound,bis(β-epithiopropyl)sulfide, 20.0 parts by weight of a polythiolcompound, 1,2-bis((mercaptoethyl)thio)-3-mercaptopropane, and 20.0 partsby weight of a polyisocyanate compound, m-xylene diisocyanate were mixedwith stirring, to which was added 0.05 parts by weight of a catalyst,dibutyltin dichloride. These were stirred and mixed under a reducedpressure of 10 mmHg for about 3 minutes to prepare a monomer compositionfor lenses. The ratio of —SH/—NCO in this composition is given in Table1.

[0060] Next, the monomer composition was cast into a glass mold forlenses (lens power 0.00 D, lens diameter 80 mm, lens thickness 1.8 mm),which had been previously prepared and equipped with a resin gasket. Themold was put into an electric furnace, and gradually heated therein atfrom 20° C. up to 120° C. over a period of 20 hours and then kept heatedat 120° C. for 30 minutes, through which the monomers were polymerized.

[0061] However, the resin thus obtained was like soft rubber, and had anoffensive smell and was much yellowed. Therefore, this was not testedfor (1) to (4).

COMPARATIVE EXAMPLE 5

[0062] 93.0 parts by weight of an epithio group-having compound,bis(β-epithiopropyl)sulfide, 6.0 parts by weight of a polythiolcompound, dimercaptoethyl sulfide, and 1.0 part by weight ofhydroxyethyl methacrylate were mixed with stirring, to which was added0.05 parts by weight of a catalyst, tetrabutylphosphonium bromide. Thesewere stirred and mixed under a reduced pressure of 10 mmHg for about 3minutes to prepare a monomer composition for lenses. The ratio of—SH/—NCO in this composition is given in Table 1.

[0063] Next, the monomer mixture was polymerized in the same manner asin Example 1. The lens obtained was visually checked for appearance, andtested for (1) to (4) as above. The results are given in Table 1. TABLE1 Epithio Polyisocyanate Polythiol Blend Ratio group-having TensileCompound Compound (-SH/-NCO) Compound Refractive Abbe's Heat Strengthwt. pts. wt. pts. by mol wt. pts. Appearance Index Number Resistance kgfStriae Example 1 DIMB BMMD 3.5 BEPS colorless 1.699 36 90 40 4.35 15.6580.0 transparent 2 DIMB MMDS/BMMD 3.0 BEPS colorless 1.699 36 92 40 5.167.91/11.93 75.0 transparent 3 BIMD BMMD 3.0 BEPS colorless 1.700 36 9053 9.30 25.70 65.0 transparent 4 HMDI MMDS/BMMD 3.0 BEPS colorless 1.69136 85 44 6.48 15.70/7.82 70.0 transparent 5 BIMD BMMD 1.80 BEPScolorless 1.697 36 89 53 12.22 20.28 67.50 transparent 6 BIMD BMMD 1.50BEPS colorless 1.696 36 92 56 13.64 18.861 67.50 transparent 7 BIMD BMMD1.25 BEPS colorless 1.695 36 96 60 15.10 17.40 67.50 transparent 8BIMD/DIMB BMMD 1.80 BEPS colorless 1.694 36 89 49 6.88/4.10 19.02 70.00transparent 9 BIMD/DIMB BMMD 1.75 BEPS colorless 1.694 36 92 547.00/4.18 18.82 70.00 transparent 10 BIMD/DIMB BMMD 1.50 BEPS colorless1.694 36 95 51 8.93/3.43 17.64 70.00 transparent Comparative 1 XDI BTG0.16 BEPS Not cured Examples 20.00 5.00 75.0 (dark brown liquid) 2 XDIDMES 0.31 BEPS Not cured 20.00 5.00 75.0 (milky, opaque jelly) 3 IPDIBTG 0.25 BEPS Not cured 15.00 5.00 80.0 (dark brown liquid) 4 XDI DMTMP1.08 BEPS Yellowed soft rubber 20.00 20.00 60.0 5 — DMES/HEMA — BEPScolorless 1.700 36 90 25 6.00/1.00 93.0 transparent

[0064] As in Table 1, the lenses of Examples 1 to 10 are all colorlesstransparent, and have a high refractive index and a high Abbe's number,and their heat resistance and tensile strength are all good.

[0065] As described in detail hereinabove, the plastic lenses forspectacles of the invention have a high refractive index and a highAbbe's number. They are transparent, and have good heat resistance,mechanical strength and impact resistance. Therefore, they are suitableto spectacles.

1-25. (Canceled).
 26. A process for preparing a material suitable for atransparent lens comprising polymerizing an epithio group-havingcompound (a), a polythiol compound (b) and a polyisocyanate compound (c)wherein the ratio (by mol) of polythiol (b) to polyisocyanate compoundis at least 1.75 calculated on the basis of —SH and —NCO groups andwherein the epithio group-having compound (a) is an aliphaticskeleton-having episulfide compound.
 27. The process of claim 26 whereinthe aliphatic skeleton-having episulfide compound isbis(β-epithiopropyl)sulfide.
 28. The process of claim 26, comprisingpolymerizing from 60 to 85% by weight of the epithio group-havingcompound (a), from 15 to 40% by weight of the polythiol compound (b) andthe balance polyisocyanate compound (c).
 29. A process for preparing amaterial suitable for a transparent lens comprising polymerizing anepitho group-having compound (a), a polythiol compound (b) and apolyisocyanate compound (c) wherein the ratio (by mol) of polythiol (b)to polyisocyanate compound is at least 2 calculated on the basis of —SHand —NCO groups, and wherein the polyisocyanate compound (c) is sulfurfree and comprises at least one sulfur-free polyisocyanate compound, andwherein the epithio group-having compound (a) is an aliphaticskeleton-having episulfide compound.
 30. The process of claim 29 whereinthe aliphatic skeleton-having episulfide compound isbis(β-epithiopropyl)sulfide.
 31. The process of claim 29 wherein theratio (by mol) of polythiol (b) to polyisocyanate compound is at least2.5 calculated on the basis of —SH and —NCO groups.
 32. The process ofclaim 30 wherein the ratio (by mol) of polythiol (b) to polyisocyanatecompound is at least 2.5 calculated on the basis of —SH and —NCO groups.33. The process of claim 29, comprising polymerizing from 60 to 85% byweight of the epithio group-having compound (a), from 15 to 40% byweight of the polythiol compound (b) and the balance polyisocyanatecompound (c).
 34. The process of claim 30, comprising polymerizing from60 to 85% by weight of the epithio group-having compound (a), from 15 to40% by weight of the polythiol compound (b) and the balancepolyisocyanate compound (c).
 35. A transparent lens for spectacles madeof a material prepared by the process comprising polymerizing an epithiogroup-having compound (a), a polythiol compound (b) and a polyisocyanatecompound (c) wherein the ratio (by mol) of polythiol (b) topolyisocyanate compound is at least 1.75 calculated on the basis of —SHand —NCO groups and wherein the epithio group-having compound (a) is analiphatic skeleton-having episulfide compound.
 36. A transparent lensfor spectacles made of a material prepared by the process comprisingpolymerizing an epitho group-having compound (a), a polythiol compound(b) and a polyisocyanate compound (c) wherein the ratio (by mol) ofpolythiol (b) to polyisocyanate compound is at least 2 calculated on thebasis of —SH and —NCO groups, and wherein the polyisocyanate compound(c) is sulfur free and comprises at least one sulfur-free polyisocyanatecompound, and wherein the epithio group-having compound (a) is analiphatic skeleton-having episulfide compound.
 37. The transparent lensof claim 35 wherein the aliphatic skeleton-having episulfide compound isbis(β-epithiopropyl)sulfide.
 38. The transparent lens of claim 36wherein the aliphatic skeleton-having episulfide compound isbis(β-epithiopropyl)sulfide.
 39. The transparent lens of claim 35,comprising polymerizing from 60 to 85% by weight of the epithiogroup-having compound (a), from 15 to 40% by weight of the polythiolcompound (b) and the balance polyisocyanate compound (c).
 40. Thetransparent lens of claim 36, comprising polymerizing from 60 to 85% byweight of the epithio group-having compound (a), from 15 to 40% byweight of the polythiol compound (b) and the balance polyisocyanatecompound (c).
 41. The transparent lens of claim 37, comprisingpolymerizing from 60 to 85% by weight of the epithio group-havingcompound (a), from 15 to 40% by weight of the polythiol compound (b) andthe balance polyisocyanate compound (c).
 42. The transparent lens ofclaim 38, comprising polymerizing from 60 to 85% by weight of theepithio group-having compound (a), from 15 to 40% by weight of thepolythiol compound (b) and the balance polyisocyanate compound (c).